Circumbinary Accretion: From Binary Stars to Massive Binary Black Holes

We review recent works on the dynamics of circumbinary accretion, including time variability, angular momentum transfer between the disk and the binary, and the secular evolution of accreting binaries. These dynamics can impact stellar binary formation/evolution, circumbinary planet formation/migration, and the evolution of (super)massive black-hole binaries. We discuss the dynamics and evolution of inclined/warped circumbinary disks and connect with recent observations of protoplanetary disks. A special kind of circumbinary accretion involves binaries embedded in "big" disks, which may contribute to the mergers of stellar-mass black holes in AGN disks. Highlights include: $\bullet$ Circumbinary accretion is highly variable, being modulated at $P_{\rm b}$ (the binary period) or $\sim 5P_{\rm p}$, depending on the binary eccentricity $e_{\rm b}$ and mass ratio $q_{\rm b}$. $\bullet$ The inner region of the circumbinary disk can develop coherent eccentric structure, which may modulate the accretion and affect the physical processes (e.g. planet migration) taking place in the disk. $\bullet$ Over long timescales, circumbinary accretion steers binaries toward equal masses, and it does not always lead to binary orbital decay, as is commonly assumed. The secular orbital evolution depends on the binary parameters ($e_{\rm b}$ and $q_{\rm b}$), and on the thermodynamic properties of the accreting gas. $\bullet$ A misaligned disk around a low-eccentricity binary tends to evolve toward coplanarity due to viscous dissipation. But when $e_{\rm b}$ is significant, the disk can evolve toward "polar alignment", with the disk plane perpendicular to the binary plane.